INVERTEBRATES 



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are at first 

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along the ventral side as two bands, each formed by the division 

 of one of a pair of pole cells at the hind end which are derived 

 from a cell ' determined ' for their formation during cleavage, and 

 in the earthworm each band 

 subsequently divides into a 

 row of mesoblastic somites. 

 These as they form become 

 hollow, and the cavity of 

 each unites with that of its 

 fellow on the other side of 

 the body to become the 

 coelom of one of the seg- 

 ments of the adult. In the 

 crayfish the cleavage (Fig. 

 526) is incomplete, but, as 

 will be seen from the follow- 

 ing account, it is of a 

 different kind from the 

 incomplete cleavage of the 

 ova of the chick and dog- 

 fish. The nucleus divides 

 till a number of daughter 

 nuclei are formed, and these 

 migrate to the surface, 

 w^here they 

 embedded in 



sheet of protoplasm which 

 encloses a central mass of 

 yolk (' centrolecithal ' 

 cleavage). Afterwards this 

 protoplasm divides into cells 

 which constitute a one- 

 layered blastoderm enclos- 

 ing the yolk mass. Thus 

 there arises a sort of blastula 

 which has no blastocoele, 

 but contains yolk. A shallow 

 invagination on one side of this, pushing into the yolk, gives rise 

 to a gastrula with a small enteron (Fig. 527). The mesoderm 

 arises as two ventral bands, though pole cells are not found : 

 it forms mesoblastic somites, whose cavities afterwards disappear, 



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